Jet combustion fuel



Patented July 14, 1953 UNITED STATES PATENT OFFICE JE1 COMBUSTION FUELJacl; M. God Qy,.Wennah, N. J., assignor to S0- cony-Vacuum Oil Company,Incorporated, a corporation of New York 7 No Drawing. Application April28, 1949,

Serial No. 90,270

2 Claims, (01. 4463) tube give rise to a reaction effect which drivesthe tube in a direction. opposite to that of the emission of the gases.The most complicated forms presently proposed consist of they samepropulsion or jet tube, plus a compressor to supply air for combustion,plus a gas turbine which extracts enough energy from the departing gasesto drive the compressor. In present commercial terms, the compressor andturbine are assembled axiallyupon a common shaft, spaced far enoughapart to permit a number of combustion cham bers to be arranged aboutthe shaft between the compressor and turbine with an exhaust tubeextending rearwardly from the turbine. In essence, the term jetcombustion, as now commonly applied, and; as used in this specification,refers to. a method of combustion wherein fuel, is continuouslyintroduced into and continuously. burned in a confined space for thepurpose of deriving power directly from the hot products of combustion.

In practice, the range of conditions over which a jet combustion devicemay operate may become quite limited for ill-chosen or ill-adaptedfuels. Even though a combustible mixture be present, the flame will beblown out if the rate of fuel feed is too far increased. Yet, high ratesof fuel feed are necessary to obtain high heat release, which is highpower delivery. Fuels with low blow-out levels can furnish only limitedpower and limited flexibility under conditions of operation.Consequently, the flame stability of a fuel is of major importance.

It has been found that flame stability of a fuel is correlated with aproperty of the fuel which is readily reproducible on a laboratorybasis, using apparatus which is simple when compared to a commercialcombustion tube, and with con siderably less procedural difficulty. Thiscorrelated property is the rate of flame propagation as measured in aBunsen-type burner, using the method of Smith and Pickering [J. Res.Natl. Bur. Stds. l7, '7 (1936)]. In this procedure, the rate of flamepropagation is measured at a number of fuel-air ratios by photographingthe flame and measuring the slope of the. flame, cone. De sirable levelsof commercial operab ility may be found in fuels having rates of flamepropagation,

of the order of 1.4 feet per second and higher.

Benzol, an excellent fuel, has a rate of flame,

propagation of about 1.6 feet per second, at a fuel mixture temperatureof 230. R, a pressure. of 1 atmosphereQand an air, flow rate, of 3,1 5pounds/hour.

The present invention is predicated upon, the

discovery that small amounts of a material, ob;

tained by vacuum distillation ofthiophene tar, boiling at -125 C. at 2mm. absolute pressure, disclosed in U. S. Patent No. 2,450,659, whenadded to a fuel, will materially improve. the com} bustion stabilitythereof.

As shown in U. S. Patent No. 2,450,659, and in copending application forLetters Patent, Serial No. 721,453, filed on Janua'rylO, 1947 ,now U. S.Patent No. 2,515,927, thiophene tar and thiophene are prepared. byseparately preheat ing sulfur and one or more normalaliphatichydrocarbons selected from the group consisting of normalbutane, normal butenes, and butadienes,

' to temperatures such that combining sulfur and the hydrocarbonmaterial will give a mixture hav: ing a temperature in excess of about450 0., mix: ing the preheated sulfur and the preheated hydrocarbonmixture, maintaining the temperature. of the mixture at a temperature inexcess of about 450 C. for a period of time of at least 0.01 secondandreducing the temperature of the mixture to less than 450 C. Along withthiophene tar. and thiophene, hydrogen sulfide and small amounts ofcarbon disulfide are also formed in the process.

It has been found in the operation of this process that the relativeproportions of sulfur. and hydrocarbon material in the charge may bevaried over wide limits. Too much sulfur, however, results in poorefficiency in sulfur utilization per pass and favors the completesulfurization of hydrocarbon material to carbon disulfide. Yet, too lowa proportion of sulfur lowers the conversion per pass and the ultimateyield by increasing the overall thermal degradation of hydrocarbonmaterial. Generally speaking, best results are obtained using a weightratio of sulfurto a hydrocarbon material varying between about 0.5 andabout 4.0, although when butenes and butadienes constitute the bulk ofthe hydrocar bon charge. the lower limit of the weight ratio may belower than 0.5. It should be observed, however, that for economicaloperation of the process, it is preferred not to use a hydrocarboncharge consisting predominantly of butadienes because of their tendencyto polymerize under the conditions of the process.

The selectivity of the reaction involved in the process for thepreparation of thiophene tars and thiophene depends primarily upon twovariables; namely, the reaction temperature at which the normalaliphatic hydrocarbon or hydrocarbons are contacted with sulfur, and thereaction time or the time durin which contact between the reactants ismaintained at the reaction temperature.

The limits of operating temperature are fixed by the kinetics of thedesired reaction and the kinetics of possible side reactions. It hasbeen found in this connection, that the reaction temperature may varybetween about 450 C. and about 760 C. and, preferably, between about 540C. and about 650 C. when butane is the predominant hydrocarbon reactantin the charge, and between about d80 C. and about 500 C. when butenesand butadienes are the predominant hydrocarbon reactants in the charge.Below the lower limit of the temperature range (about 450 C.), thereaction is so slow as to require a large throughput of sulfur and ahigher ratio of hydrocarbon recycle for a fixed amount of end product,thereby detracting from the economics of the operation. Above the upperlimit of the temperature range, the secondary reaction of degradation ofhydrocarbon material in the charge takes precedence, thereby decreasingthe yield of desired product. In addition to this, high temperaturesfavor the formation of carbon disulfide. It must be noted, also, that atthese high temperatures corrosion problems are at a maximum, corrosionincreasing perceptibly with increasing temperature.

It has also been found, in connection with this process, that theoptimum reaction time depends upon the temperature employed. In general,other variables remaining constant, the lower the temperature, thelonger the reaction time. The reaction or contact time and the reactiontemperature are somewhat fixed, one in relation to the other, by thedegree of degradation of the hydrocarbon material in the charge and bythe effect of formation'of undesirable products which may be tolerated.Thus, too long a contact time at high temperature results in severecracking of the hydrocarbon material in the charge. The reactionproceeds with extreme speed, the only apparent limitation being therapidity with which heat can be supplied to the reaction mixture. Thereaction is highly endothermic, requiring by experimental measureapproximately 28,000 calories per gram molecular weight of thiopheneproduced from normal butane. The lower limit of the range of reactiontime is fixed, therefore, by the engineering problem of heat transferand by mechanical limitations such as allowable pressure drop across thereactor. Relatively long reaction times at temperatures in theneighborhood of the lower limit of the temperature range result ininsufficient reaction. Accordingly, it has been found that for bestresults the time of reaction is fixed by the reaction temperature.

In view of the foregoing, the criteria to be used in determining optimumoperating temperatures within the range of 450 C. to 760 C. depend onthe degree of conversion desired commensurate with operating costs, suchas heat input and equipment cost, bearing in mind that within limits,the shorter the reaction time, and accordingly the higher thetemperature, the larger the amount of end product which can be realizedfrom a unit of given size per day.

While the relationship between the temperature of reaction and reactiontime is not peculiar to the present process, it has been found thatthiophene tar and thiophene may be produced by reacting sulfur and theaforesaid l-carbon hydrocarbons at a temperature between about 450 C.and about 760 C. for a period of time selected to minimize the yields ofsecondary reaction products such as carbon disulfide, coke-likematerials and the like at the selected temperature. Under suchcondtions, when operating continuously with a reactor coil of suitablesize and at a practical charge rate, it has been found that the lowestpractical limit of the time of reaction is of the order of 0.01 secondat about 760 C. The upper practical limit of the reaction time, othervariables remaining constant, will correspond to the lower limit of thereaction temperature and may be of the order of several seconds.

Separate preheating of the hydrocarbon reactant and sulfur and quenchingof the reaction mixture are necessary for achieving the somewhat closecontrol of the reaction time at a given reaction temperature. This isvery important in producing the specific reaction products, thiopheneand thiophene tar. It is suspected that a number of reactions occur uponcontacting the hydrocarbon reactant with sulfur. In this connection, thefollowing should be noted: cracking of the hydrocarbon reactantdestroying the 4-carbon atom chain structure (said 4-carbon atom chainstructure being a prerequisite for the formation of thiophene),formation of thiophene tars high in sulfur and formation of carbondisulfide. These reactions compete with one another. It has been foundthat the rates of the formation of lighter hydrocarbons and of theformation of carbon disulfide are somewhat slower than those requiredfor the formation of thiophene and thiophene tar. Accordingly, propercontrol of the reaction time at a given reaction temperature, achievedby separate preheating, mixing, heating at a given temperature for acorresponding period of time, and quenching is necessary to produce highyields of thiophene and thiophene tar with limited yields of carbondisulfide, coke-like materials, and fixed gases, due to limiteddecomposition of the hydrocarbon product. The rate of the reactionproducing thiophene tar is fairly close to that required for theformation of thiophene, and the yields of thiophene tar and or"thiophene are approximately the same. Upon standing, the thiophene tarseparates from the other products and a separation can be made bydecantation or other suitable separating means.

In this process the reaction is effected preferably at atmosphericpressure or under suflicient ressure to cause the flow of the reactantsthrough the reactor and auxiliary system under the desired reactionconditions. Tests have shown that the conversion per pass and ultimateyield of thiophene decreases with increasing pressure. However, even atappreciable pressures, thiophene and thiophene tar are, never theless,produced in substantial amounts.

Vacuum distillation of the above described thiophene tars is adestructive distillation process in which the charge, probablydisulfides, polysulfides, etc., is decomposed during the heating processinto distillable liquids and hydrogen sulfide. Vacuum distillation ofthe original tar and subsequent vacuum fractionation of the distillateso obtained yields two distinct fractions, a lower boiling material anda higher boiling material constituting the compound used in thisinvention. There is nothing critical in the vacuums employed duringthese distillations.

During the course of the aforesaid vacuum distillation, hydrogen sulfideis evolved, giving rise to frothing and bumping of the tar. Theseundesirable conditions have been overcome, however, by resorting to anyone of several modifications: Smoother operation is realized by eating acapillary tube in the distillation vessel so that its lower end islocated below the surface of the boiling tar and directing a stream ofinert gas, such as carbon dioxide, nitrogen, or the like, through thetube and thus through the boiling tar. Another expedient involves firstevacuating the distillation vessel at room temperature to degas the tartherein and thereafter slowly increasing the temperature of the tar.Hydrogen sulfide evolved from the tar during the distillation is readilyremoved by scrubbing the evolved gases by passing through towers filledwith acid-absorbing media, such as soda lime, sodium hydroxide pellets,etc. This absorption of hydrogen sulfide protects the mechanical movingparts of the pump used to obtain the desired vacuum and hence is highlydesirable. However, if a steam ejector system is used to obtain vacuum,the preliminary absorbing step may be omitted, since in this case, thehydrogen sulfide will be exhausted to the atmosphere.

It has been found that maximum distillation efiiciency can be attainedby keeping the pressure below 10 millimeters and preferably below 2millimeters of mercury. If the pressure is permitted to rise to theorder of 10 millimeters of mercury, the temperature must necessarily beincreased for distillation to occur at a reasonable rate and ultimatelythe rate of decomposition with evolution of hydrogen sulfide becomes toorapid to maintain an appreciable vacuum. When the temperature of theinitial distillation rises to the neighborhood of 250 C., the tar has atendency to polymerize and coke. Accordingly, the temperature of thisdistillation should be maintained between about 150 C. and about 250 C.and, preferably, between about 175 C. about 190 C. to attain a maximumyield of red, oily distillate. Under the above specified conditions oftemperature and pressure, approximately -50 per cent of the initialcharge of thiophene tar is distillable.

Subsequent vacuum fractionation of the red, oily distillate isordinarily carried out at pressures below 10 millimeters of mercury and,preferably, at 4 millimeters of mercury or below. Such re-distillationyields two distinct fractions, a low-boiling fraction (4045 C. at 2millimeters) constituting 60-85 per cent of the initial distillate and ahigh-boiling fraction (120-125 C. at 2 millimeters) constitutingapproximately 1540 per cent of the initial distillate.

The following example will serve as an illustration:

EXAlWPLE A mixture containing 30 per cent by volume of 1,3-butadiene and70 per cent by volume of normal butane was charged into a preheater at arate of grams per minute and heated to a temperature of 590 C. Sulfurwas charged to a separate preheater at a rate of 28 grams per minute andheated to a temperature of 590 C.

constructed of 27 per cent chromium stainless steel, maintained at atemperature of 650 C. The reaction product was quenched with a waterspray passed through a small Cottrell precipitator to remove tar mistand scrubbed through a hot counter-current caustic tower. Liquid productwas condensed and'separated in a water cooler and ice trap and theresidual gas was metered. Of the hydrocarbon material charged, 49 percent was converted to a liquid product and tar. Fractionation of aportion of the liquid product after removal of C'i-hydrocarbons andlighter constituents showed the following composition:

Per cent Carbon disulfide 9.0 Thiophene 80.5 Residue (mostly thiophene)10.5

One hundred parts by Weight of the thiophene tar were vacuum distilledin a distillation vessel immersed in a heating bath. l/Vhile warming thetar, a stream of nitrogen was bubbled through the heavy liquid until thetar was degassed. A scrubbing tower for removal of hydrogen sulfide anda Dry Ice-acetone condenser for removal of light liquids were connectedin series before the vacuum pump. The bath. temperature was allowed torise slowly and then was maintained at 175-200 C. A pressure of 1millimeter of mercury was initially obtained but this gradually roseupon prolonged heating of the tar until a maximum pressure of 10millimeters of mercury was reached. The product consisted of 226 partsby weight of a red, oily distillate boi ing between C. and 175 C. Theyield of said distillate, based on the weight of tar, was 45.3 per cent;

The red oil was then vacuum-fractionated at a pressure of 2 millimetersof mercury, whereupon the following fractions were obtained:

- Wei ht 13011111 Fraction 7 Point Range gggga g atzmm" Original Tar)Material boiling within the range of fraction 2 was shown to have thefollowing properties:

Molecular Weight i 157 Carbon 34. 46

Hydrogen 2. 60 ulfur G2. 0

Molecular formula O-(HiSl Refractive index, 2 0 C 1. 70

opecific gravity 25 C. 1.446

Color Deep Red sired, although, in general, no outstanding re- The twostreams were sent through a mixing sults seem to occur. conceivably andwithin the scope of the present invention, the jet fuel additivescontemplated herein may be marketd or procured as concentrates, vi z.,jet combustion fuls containing upwards of 10% and up to 49% by weight ofthe additive. These concentrates are subsequently added to a jetcombustion fuel in such proportions as to produce the effectiveconcentration of additive in the fuel desired, i. e., a suificientamount to improve the jet combustion properties of the jet combustionfuel.

The jet combustion fuels of the present invention may contain othermaterials or additives for improving other characteristics thereof.Carbon deposition-reducing additives, gum inhibitors, and starting aidsare mentioned by way of non-limiting examples of other additives whichmay be present in the jet combustion fuels of the present invention.

The addition of the thiophene tar fraction (fraction 2 as hereinbeforeindicated) may be made to fuels of a fairly wide variety of boilingranges, specific gravity, etc. Suitable base fuels for use in accordancewith this invention include .1

those having the character of light gasolines up to those having thecharacter of gas oils: Synthetic fuels, such as those manufactured bythe Fischer-Tropsch process, can be used, as can be fuels derived fromcoal or wood distillation. It is also contemplated to add thesecombustion improving additives to liquid alcohols or combinations ofalcohols with other base fuels. The preferred fuel is, however, ahydrocarbon distillate fuel boiling within the range of about 100 F.(37.8 C.) to about 600 F. (315.6 C.).

The physical characteristics of a few examples of suitable base fuelsare given hereinafter for illustrative purposes:

3. Hydrocarbon distillate fuel Boiling range 320-470 F.

Gravity 365 A. P. I.

Freezing point"- Below -76 F.

Sulfur 0.035 by weight Bromine No 1 Aromatics 12.1 by volume Viscosity1.48 centistokes, at 100 F.

The following examples are given for the purpose of illustrating thepresent invention and for indicating the advantages thereof. It must beclearly understood, however, that these examples are non-limiting. Itwill be appreciated by those skilled in the art that numerous types ofjet combustion fuels, other than the standard reference fuel describedhereinafter, may be used for the purpose contemplated herein.

In these tests a reference fuel was used which was substantially2,2,4-trimethylpentane, commonly known as S-reference fuel. The rate offiame propagation using the standard reference fuel was compared withthat of a blend of the S-reference fuel with 0.5% by weight of theadditive material hereinbefore disclosed, in accordance with theprocedure of Smith and Pickering also hereinbefore disclosed. The fuelmixture was maintained at a temperature of about 230 F., under apressure of 1 atmosphere, and the air flow rate was controlled at 3.15pounds/ hour.

Table Rate of flame propagation reference fuel 1.45 ft./sec. 99.5%reference fuel+0.5% C4H4S3 additive material 1.50 ft./sec.

It will be seen that by adding as little as 0.5% of the additivematerial to the fuel, an increase of approximately 4% in rate of flamepropagation can be obtained, which, as indicated hereinbefore, means asubstantial improvement in combustion stability.

This application is a continuation-in-part of copending applicationSerial No. 26,477, filed May 11, 1948, now abandoned, which in turn is acontinuation-in-part of application Serial No. 744,024, filed April 25,1947, now abandoned.

I claim:

1. A liquid fuel capable of being utilized in jet combustion mechanisms,which comprises a hydrocarbon distillate having an initial boiling pointof about 40 C. and a final boiling point of about 315 C. and boilingsubstantially continuously between said boiling points, and betweenabout 0.1 per cent and about 2.0 per cent by weight of a fractionboiling at l20-125 C., at a pressure of 2 mm. of mercury, obtained bythe process which comprises separately preheating sulfur and aGil-hydrocarbon selected from the group consisting of normal butane,normal butenes, and butadienes, to temperatures such that combining saidsulfur and said hydrocarbonwill give a reaction mixture having atemperature falling within the range varying between about 450 C. andabout 760 C.; mixing the preheated sulfur and the preheated hydrocarbon;reacting said preheated sulfur with said preheated hydrocarbon at areaction temperature falling within the range Varying between about 450C. and about 760 C. for a period of time selected to minimize the yieldsof hydrocarbons containing less than four carbon atoms per molecule andcarbon disulfide at said reaction temperature, to yield a mixturecontaining thiophene tar; immediately reducing the temperature of themixture containing said thiophene tar to a temperature of less thanabout 450 C. separating the thicphene tar from said mixture; subjectingsaid thiophene tar to vacuum distillation at temperatures Varyingbetween about 150 C. and about 250 C. and at a pressure of less thanabout 10 mm. of mercury, to produce a first distillate; and subjectingsaid first distillate to a vacuum fractionation at a pressure of lessthan about 10 mm. of mercury, to produce said fraction boiling at -125C., at a pressure of 2 mm. of mercury.

2. A liquid fuel capable of being utilized in jet combustion mechanisms,which comprises a hydrocarbon distillate having an initial boiling pointof about 40 C. and a final boiling point of about 315 C. and boilingsubstantially continuously between said boiling points, and betweenabout 0.1 per cent and about 10.0 per cent by weight of a fractionboiling at 120-125 C., at a pressure of 2 mm. of mercury, obtained bythe process which comprises separately preheating sulfur and aCi-hydrocarbon selected from the group consisting of normal butane,normal butenes, and butadienes, to temperatures such that combining saidsulfur and said hydrocarbon will give a reaction mixture having atemperature falling within the range varying between about 450 C. andabout 760 0.; mixing the preheated sulfur and the preheated hydrocarbon;reacting said preheated sulfur with said preheated hydrocarbon at areaction temperature falling within the range varying between about 450C. and about 760 C. for a period of time selected to minimize the yieldsof hydrocarbons containing less than four carbon atoms per molecule andcarbon disulfide at said reaction temperature, to yield a mixturecontaining thiophene tar; immediately reducing the temperature of themixture containing said thiophene tar to a temperature of less thanabout 450 C.; separating the thiophene tar from said mixture; subjectingsaid thiophene tar to vacuum distillation at temperatures varyingbetween about 150 C. and about 250 C. and at a pressure of less thanabout 10 mm. of mercury, to produce a, first distillate; and subjectingsaid first distillate to a vacuum fractionation at a pressure of lessthan about 10 mm. of mercury, to produce said fraction boiling at120-125 0., at a pressure of 2 mm. of mercury.

JACK M. GODSEY.

References Cited in the file Of this patent UNITED STATES PATENTS Number

1. A LIQUID FUEL CAPABLE OF BEING UTILIZED IN JET COMBUSTION MECHANISMS,WHICH COMPRISES A HYDROCARBON DISTILLATE HAVING AN INITIAL BOILING POINTOF ABOUT 40* C. AND A FINAL BOILING POINT OF ABOUT 315* C. AND BOILINGSUBSTANTIALLY CONTINUOUSLY BETWEEN SAID BOILING POINTS, AND BETWEENABOUT 0.1 PER CENT AND ABOUT 2.0 PER CENT BY WEIGHT OF A FRACTIONBOILING AT 12/-125* C., AT A PRESSURE OF 2 MM. OF MERCURY, OBTAINED BYTHE PROCESS WHICH COMPRISES SEPARATELY PREHEATING SULFUR AND AC4-HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF NORMAL BUTANE,NORMAL BUTENES, AND BUTADIENES, TO TEMPERATURES SUCH THAT COMBINING SAIDSULFUR AND SAID HYDROCARBON WILL GIVE A REACTION MIXTURE HAVING ATEMPERATURE FALLING WITHIN THE RANGE VARYING BETWEEN ABOUT 450* C. ANDAOUT 760* C.,; MIXING THE PREHEATED SULFUR AND THE PREHEATEDHYDROCARBON; REACTING SAID PREHEATED SULFUR WITH SAID PREHEATEDHYDROCRBON AT A REACTION TEMPERATURE FALLING WITHIN THE RANGE VARYINGBETWEEN ABOUT 450* C. AND ABOUT 760* C. FOR A PERIOD OF TIME SELECTED TOMINIMIZE THE YIELDS OF HYDROCARBONS CONTAINING LESS THAN FOUR CARBONATOMS PER MOLECULE AND CARBON DISULFIDE AT SAID REACTION TEMPERATURE, TOYIELD A MIXTURE CONTAINING THIOPHENE TAR; IMMEDIATELY REDUCING THETEMPERATURE OF THE MIXTURE CONTAINING SAID THIOPHENE TAR TO ATEMPERATURE OF LESS THAN ABOUT 450* C.; SEPARATING THE THIOPHENE TARFROM SAID MIXTURE; SUBJECTING SAID ATHIOPHENE TAR TO VACUUM DISTILLATIONAT TEMPERATURES VARYING BETWEEN ABOUT 150* C. AND ABOUT 250* C. AND AT APRESSURE OF LESS THAN ABOUT 10 MM. OF MERCURY, TO PRODUCE A FIRSTDISTILLATE; AND SUBJECTING SAID FIRST DISTILLATE TO A VACCUMFRACTIONATION AT A PRESSURE OF LESS THAN ABOUT 10 MM. OF MERCURY, TOPRODUCE SAID FRACTION BOILING AT 120-125* C., AT A PRESSURE OF 2 MM. OFMERCURY.